Temperature measurement is important to studies of heat generation and transfer processes in a wide range of engineering systems. However, the feature sizes of many engineering systems, such as microelectronic, optoelectronic, and micromechanical systems, have been reduced down to length scales as small as tens of nanometers and continue to decrease. Experimental studies of the nano-scale thermal processes involved in such systems are not possible without high spatial resolution temperature measurement techniques.
Such existing techniques can be grouped into three categories—scanning probe based temperature mapping techniques, optical temperature mapping techniques, and thin coating methods. Scanning probe based techniques, such as scanning thermal microscopy (SThM), are techniques which employ a temperature sensing scanning probe microscopy (SPM) tip to scan across the sample while the temperature signal is collected to form a thermal image. Although SThM can achieve lateral resolution on the order of 50 nm, probing the sample by SThM requires contact and heat diffusion between the probe and the sample surface. This causes an associated heat diffusion delay, topography-related artifacts in the thermal images, artifacts resulting from heat exchange through the sides of the probe (rather than just the tip) and thus perturbs the original micro-scale temperature distribution of the sample.
Optical techniques, such as infrared thermography, fluorescence thermography, thermoreflectance microscopy, optical interferometry, and micro-Raman thermography, address these issues with optical non-contact temperature mapping, but the spatial resolution of optical thermometry is typically limited by the wavelength of the radiation employed and cannot reach the typical feature sizes of current nano-engineered systems. Although near field scanning optical microscopy breaks the diffraction limit of optical systems and provides higher spatial resolution by employing an optical fiber or an SPM tip with an ultra-small aperture close enough to the sample surface, it is still subject to the typical draw backs of SPM-based temperature mapping techniques. Also, since the tip is very close to the surface, the tip itself can undergo heating from the sample and suffer a change in geometry, affecting the reflected signal.
Thin coating methods, such as liquid crystal thermography, use the color change of a liquid crystal coating deposited on the sample surface for temperature indication. However, issues with the thermal conductivity, heat capacity, and non-uniformity of the liquid crystal coating may lower the accuracy of the temperature measurement. Also, the thin coating may perturb the temperature distribution of the sample, and the spatial resolution of thin coating methods is still limited by the long optical wavelength.